The aerospace industry has developed various powder metalurgy techniques over the last several decades to produce near-net shapes in engine and airframe structures to effect savings in finishing labor and material. Such techniques directly produce a net shape or near net shape from raw material. Powder metallurgy techniques offer the potential of producing compressor wheels with various combinations of aluminum, steel, and titanium in the blade and hub. The capability of using different alloys to construct the various regions of the compressor permits tailoring of the composition and processing for each region to the critical requirements of that location.
The principal processes heretofore utilized have been: (a) cold isostatic pressing and sintering, and (b) cold isostatic pressing, sintering, followed by hot isostatic pressing.
One of the attributes desired in a mold for cold isostatic pressing is that the mold be sufficiently flexible to accommodate the volume change associated with compaction. On the other hand, the mold must be sufficiently rigid to preclude penetration of the powder particles, as this makes it difficult to strip the envelope from the compact and is a potential source of contamination. Moreover, the mold must have the ability to withstand, when unsupported, a moderate internal pressure without bursting as well as the capability of being easily and reliably sealed.
Several materials meet the above requirements in varying degree. For example, natural and synthetic rubber, silicone elastomers, and PVC are commonly employed. The selection of an envelope depends upon the number of components required, as well as their shape and size. The type of operation in which the powder is completely encased in an envelope and the whole assembly subjected to isostatic pressing is known as "wet-bag" tooling. "Wet bag" tooling is suitable for producing complex forms or for short production runs. Filling is done with the mold removed from the pressure chamber. After filling, the elastomeric mold is usually de-aired for better compaction, sealed and placed into the pressure chamber. If the chamber is large enough, several molds can be pressed at the same time.
Isostatic compaction tends to result in increased and uniform density at a given compaction pressure. This is a consequence of more uniform pressure distribution within the compact and the absence of die-wall friction.
Isostatic pressing was first used to prepare billets of refractory metal powders as disclosed in U.S. Pat. No. 1,081,618, issued Dec. 16, 1913, to H. D. Madden, entitled "Process of Preparing Billets of Refractory Materials". The process was further used and developed to make refractory metal tube as disclosed in U.S. Pat. No. 1,226,470, issued May 15, 1917, to W. D. Coolidge, entitled "Refractory Metal Tube". However, it was not used in significant production until the late 1930's at which time it was utilized in the production of blanks for the insulators of spark plugs as disclosed in U.S. Pat. No. 1,863,854, issued June 21, 1932, to B. A. Jeffery, entitled "Method and Apparatus for Shaping Articles".
Isostatic pressing has gained rapidly since 1955 as the result of a great deal of research and development by Battelle Memorial Institute. In addition, a number of patents have issued on specific applications of the process. For example, U.S. Pat. No. 4,063,939, to Weaver teaches use of a mold to define the final shape of a powder metal material. Powdered metal is placed in the mold along with prefabricated blades and further processed to become a metallurgically bonded structural part. Hot isostatic pressing of the part is used for final densification to approach 100% density and diffusion bonding of the blades to the powder hub.
It is to be noted, however, that the Weaver process takes the mold and powder to high sintering temperatures thereby giving the part shape and strength. Moreover, the Weaver process uses a rigid mold.
U.S. Pat. No. 4,097,276 to Six, teaches use of a container filled with powder and prefabricated blades similar to Weaver. The Six sealed assembly is heated and isostatically pressed simultaneously to get shape and structure where Weaver uses sintering first, for shape and then hot isostatic pressing secondly for structure.
Catlin, U.S. Pat. No. 3,940,268, teaches the use of a rigid container for the shaping of turbine wheels which requires hot isostatic pressing for shape and structure. The Catlin process is very similar to the Six disclosure except that Catlin specifically teaches the use of metallic blades.
Webb, U.S. Pat. No. 3,000,081, discloses a process distinctly different from the aforesaid patents in that molten metal is poured around pre-fabricated blades or metal is hot forged to flow around the roots of pre-fabricated blades.
Bloomberg, U.S. Pat. No. 2,897,318, teaches a method using a casting process for locating and bonding pre-fabricated blades.
More recently, it was found that cold isostatic pressing plus sintering of Ti-6Al-4V titanium alloys could produce from 90-98% dense compacts having tensile strengths above 110 ksi with acceptable elongation at the higher densities. Controlled vacuum sintering is required for parts that have been cold-compacted. Sintering takes place between 2250.degree. F. and 2450.degree. F. usually in two to four hour cycles.